VFD Installation Considerations - McNaughton-McKay · PDF filejacketed, shielded type tray...

Copyright © 2013 Rockwell Automation, Inc. All Rights Reserved.Rev 5058-CO900D

VFD Installation ConsiderationsLoad Side

Copyright © 2013 Rockwell Automation, Inc. All Rights Reserved.

Agenda

Motor Bearing Problems

Common Mode Currents

Reflective Wave

Cable Charging

Cable Recommendations

Motor Recommendations

Copyright © 2013 Rockwell Automation, Inc. All Rights Reserved. 3


1. Must be capable of withstanding voltage spikes of x2 with rise time of > .1 µS.

2. “Inverter Duty Rated” or “Premium” usually specs a better THERMAL rating, not insulation breakdown. Don’t be confused by thermal ratings and voltage ratings.

3. Motor manufacturers are improving their design to increase their insulation. Many motor manufacturers offer .5-600+ HP motors (1600, 1850 volt category) that meets the NEMA MG1 specification.

4. Motors that meet new NEMA MG1, Part 31, 480 and 600 690 volt applications provides assurance of insulation robustness.

NEMA MG1, Part 31 titled ”Definite-purpose Inverter-fed Motors”

Is an industry standard that deals with PWM drives and motors. Part 31of NEMA MG 1 states that motors rated for operation at 600V or less should be capable of withstanding voltage peaks up to 1600V with a rise timethat is not less than 0.1μS.


NEMA Types of Motors

There are other motors that meet 1600V and 1850V voltage spike ratings


Size Drives to Motor FLA not HP


Type Cause Action1 (resettable) Internal electronic overload

trip.• Reduce load so drive

output current does not exceed the current set by OL parameter [Motor OL Current].

• Verify [Boost Select] setting.

• Size Drive properly if undersized

• Increase ramp rates• Tune drive to motor or put

in V/Hz mode and adjust boost settings

Excessive load on motor, Motor stalling

#4 Common Fault - F07 Motor Overload


#3 Common Fault – F12 HW Over Current The drive output current has exceeded the hardware current limit. >220% of drive rating. Output short circuit Energizing output contactor after the drive has started Incorrect motor tuning and data Shock load on the motor For smaller drives – long cable length causing higher cable charging currents Incorrect PI loop set up (large error or incorrect gain settings) Wet or damp conditions with motor, output cables or any connections


Agenda



Reflective Wave

Cable Charging




Recommended Motor Cables

There are many more considerations for motor cables on a VFD than on input power cables. Among these are:• Distance• Shielding• Insulation type• Moisture conditions• CIV susceptibility

While THHN will work in most cases, Rockwell recommends a 3-conductor, with ground, PVC jacketed, shielded type tray cable with XLPE conductor insulation.

This cable is superior to loose THHN wire in most applications and significantly reduces common mode noise.

Is THHN or SO cable good enough?


Example of Recommended VFD Cable

VFD CONNECTORS

VFD CABLESTray and Armor Constructions(comes in sizes 16-750MCM)

http://www.servicewire.com

Another good example of recommended cable is Belden 295xx (xx determines gauge).This cable has four (4) XLPE insulated conductors with a 100% coverage foil and an 85% coverage copper braided shield (with drain wire) surrounded by a PVC jacket.


Belden VFD Cable White Paper

http://www.belden.com/pdfs/techpprs/capwmtp.pdfCovers the advantages and disadvantages of IGBT based drives and effects on cables


VFD Cable Selection


Motor Cable Length is Important


Agenda



Reflective Wave

Cable Charging




Capacitive Coupling Phenomenon

What is Capacitive Coupling?1. In any given motor cable there will be a

certain amount of distributed stray capacitance.

2. Every time the drives’ DC bus voltage switches at the carrier (or PWM) frequency it causes current to conduct through this capacitance.

3. These capacitive current spikes then get reflected back to the drive and measured by it’s current feedback circuitry.

4. THIS IS ALSO REFERRED TO AS “CABLE CHARGING CURRENT”.

5. Causes Instantaneous Hardware Overcurrent (F12) faults in the VFD.

VFD cable can actually make this worse by the multiple shields and conductors.

Capacitor Construction is similar


Capacitive Coupling Currents

Cable Charging adds:• THHN 0.5Amp/ 100Ft.• SO Cable 0.8Amp/ 100Ft.• Armored VFD Cable 1.0Amp/ 100Ft.

General rule of thumb:

Biggest problem is for drives under 5 Hp and long motor leads


Capacitive Coupling Solutions

Capacitive Coupling Solutions

1. Oversize VFD’s to handle the extra current needed. 2. Shorter cables (less capacitance).3. Try newer specialty cables that specifically address this.4. Lower Carrier Frequency in VFD


Agenda



Reflective Wave

Cable Charging




Reflected Wave Waveforms

VLL @ Drive

VLL @ Motor

Drive

Motor

What is Reflected Wave?• They are the high frequency

voltages on top of the DC bus voltage that dealt with at the motor and are seen at the cable, motor and back at the VFD.

• Reflected waves occur due to a mismatch in impedance between the motor cables and the motor.

• Caused by the voltage rise time (dv/dt) of the IGBT switching and made worse by long cable lengths.


Reflected Wave Example


Reflected Waves exists here…

Power Source AC Drive Motor

Reflected Waves

M


The Cause of Reflected Wave

VFD rapid PWM output voltage rise times (dv/dt) The cable impedance does not match the motor

impedance. Long Cable Lengths

– Voltage reflection at “long” cable length WILL occur !!

– High voltage spikes at motor!!! Only applies to 460Vac and higher motors

Physics! Agh!Just tell me what I need to do!

If you really want to know the physics of it all read supporting IEEE papers @ http://www.ab.com/drives/techpapers/ieee/ieee.html


What damage does Reflected Wave do?

• First turn of winding failures (caused by Corona Inception Voltage - CIV)

• Seek and Find Any Insulation Voids (particularly in THHN wire).

• Peak withstand voltage for slow risetimes(for example, > 6 uS) was determined by the breakdown strength of the magnet wire

• Peak withstand voltage for fast risetimesand higher fc causes corona failure mechanism of the magnet wire insulation

• IGBT technology creates greater stresses on the motor’s insulation system than previously seen with older style switching devices (SCR, BJT, etc)


What Can You Do About Reflected Wave?

Possible Solutions for Reflective Wave Problems:

1. Keep motor lead lengths as short as possible. See User Manuals for max distances allowed.

2. Specify and buy inverter duty insulated motors (meeting NEMA MG1, Part 31).

3. Install a motor “protection” device where needed. There are motor terminators, load reactors, etcavailable for protection of existing motors.

4. Lower carrier frequency in the VFD. Default is typically 4kHz. Can go as low as 2kHz.


Agenda



Reflective Wave

Cable Charging




What is Common Mode Noise ?

High frequency noise generated by the fast dv/dt transitions, cause havoc on electrical systems. Common Mode Noise is a type of electrical high frequency noise induced on signals with respect to ground.

The path of the noise typically travels the path of least resistance back to its source.


Common Mode Noise exists here…

Power Source AC Drive Motor

Common Mode Noise

M


What can Common Mode Noise do?

Common Mode Noise can:

• Interfere with control signals (PLC, encoders, etc)• Disrupt sensitive equipment like sensors, communication

systems and computers.• Disrupt contacts on electromechanical equipment• Interferes with radio reception


Solutions for Common Mode Noise?

1. Contain the noise with cabling (armored VFD cable with appropriate connectors are great at this)

2. Complete ground connection for motor and drive. Don’t connect signals grounds to the same point.

3. Don’t’ connect motor grounds to green/yellow terminal blocks (these are connected to panel backplane).

4. Adding common-mode chokes or ferrite cores at VFD (1321-M) or LC filter at the motor (1203-RWR).

5. Proper panel layout is very important.


Panel Layout is Important

Separate power and control wire, wireway, and all equipment by at least 1 ft.


Recommended Panel Installation

Powerflex 750-series

Powerflex 520-series


Heatsink temperature exceeds 100% of Drive Temp (90 Degrees C). High ambient temperature Fan is no longer working Debris blocking the heatsink Excessive load Higher PWM frequency being used Installation Wrong

#5 Common - F08 Heatsink Overtemp


Agenda



Reflective Wave

Cable Charging




Bearing Currents

What are Bearing Currents?

Bearing currents are a result of the common mode noise generated by the VFD high speed switching. When there is no path for the high frequency current to flow, it finds its way onto the shaft of the motor and causes slow damage to bearings.

Bearing damage is more likely to occur in applications where the coupling between the motor and load is not electrically conductive (such as belted loads), when the motor is lightly loaded, or when the motor is in an environment with ionized air.


Bearing Currents Cause Mechanical Damage

Bearing damage progresses ….

Noisy & vibrations are symptoms - lubrication fatigue and mechanical wear yields to ultimate failure

Fluting


Currents Can Cause Bearing Pitting


Solutions for Bearing Currents

• Grounding Brushes: to provide an alternate path for thecurrent flow (and to reduce the peak voltage seen acrossthe bearing) may be used to provide relief. A caution formotors built with one insulated bearing would be to avoidputting a grounding brush at the end where the insulatedbearing is.

• Insulated Bearings: may be provided by either using“ceramic” balls, or by insulating the bearing from themotor end shield.

• Motor terminator filters can help resolve this.


Publications

Wiring and Grounding Guidelines for PWM Drivesdrives-in001_-en-p

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

1321 Power Conditioning Products Technical Data1321-td001_-en-phttp://literature.rockwellautomation.com/idc/groups/literature/documents/td/1321-td001_-en-p.pdf

Drives Engineering Handbook1300-DEH-10http://literature.rockwellautomation.com/idc/groups/literature/documents/at/drive-at001_-en-p.pdf

AC Drive Installation ConsiderationsDRIVES-IN003A-EN-P



http://literature.rockwellautomation.com/idc/groups/literature/documents/td/1321-td001_-en-p.pdf

http://literature.rockwellautomation.com/idc/groups/literature/documents/at/drive-at001_-en-p.pdf



Summary1. Use NEMA MG1, Part 31 rated motors2. Use shielded motor cable (Belden cable with XLPE/XHHW insulation

used on conductors - Avoid vinyl coated PVC THHN)3. Oversize drives or shorten runs on small VFD’s to prevent cable

charging.4. Use output reactors or motor filters to prevent reflected wave at motor.

Also, use inverter rated motors.5. Lower carrier frequency if reflected wave or cable charging is a

problem.6. Bond Motor frame to Drive frame and transformer ground system to

prevent common mode noise.7. Add common mode chokes or cores to prevent common mode noise.8. Separate sensitive equipment from power equipment in panels9. Use shaft grounding brushes or insulated bearings if common mode

noise is damaging bearings.


#1 Common Fault – F81 DPI CommLoss

* Communications are important to drives integration – will be covered in the afternoon


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Rev 5058-CO900D

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VFD Installation Considerations - McNaughton-McKay · PDF filejacketed, shielded type tray...

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